Ziv, Baruch (Israeli Meteorological Service)
During the winter season, the northern Africa and the middle east are sometimes subjected to heavy and widespread rain, amounting to several tens of mm within a few hours, resulting in harmful floods. Analyses of the upper tropospheric wind field and satellite imageries indicate the association of these situations with pronounced wavy deformations in the subtropical jet-stream, regularly stable with respect to such disturbances.
Analysis of the moisture field and the transverse circulations along the jet in such situations, done by the use of the ECMWF initialized data, indicate some common characteristics. The moisture originating from the tropics is advected toward the north by the ageostrophic wind across the exit region of the jet, ahead of the trough. This moisture is then advected along the jet and condenses as the result of ascending air near the inflection point of the jet, between 20░-30░N latitudes. The intensive moisture supply together with the instensive ascendance due to the pronounced curvature of the jet, explain the extraordinary amount of precipitation, sometimes over the annual average in these arid regions. This thesis has been verified for several events and demonstrated here by a case study.